1. Field of the Invention
This invention relates to a mold for press-molding glass elements such as lenses and prisms.
2. Description of the Prior Art
Optical glass lenses are in the trend of aspherical design which can achieve simplification of lens compositions of optical devices and reduction of weight of lens elements simultaneously. The aspheric lenses are low in processability and unsuited to mass production in the optical polishing method used in the conventional optical lens manufacturing process. Therefore, direct press-molding method is highly expected as its measure.
The direct press-molding method uses a pair of mold elements each having a desired high-quality and high-precision aspheric surface for molding therebetween a preheated (softened) lump of glass. This method does not require further preparatory operations such as polishing, lapping and any other processes. Therefore, in the direct press-molding method, it is required that the press-molded optical glass lens should have an excellent quality and precision.
To satisfy this requirement, the mold is required to be chemically inactive to the glass at high temperatures, resistant to scratch or other injury on the mold surfaces, and resistant to breakage due to thermal impact. To achieve these requirements, such molds have been studied that uses a super hard alloy mainly consisting of tungsten carbide (WC) or a compound material such as a cermet mainly consisting of titanium carbide (TiC), alumina (Al.sub.2 O.sub.3), titanium nitride (TiN) or chromium carbide (Cr.sub.3 C.sub.2) for forming a base member, and a noble metal layer for forming a mold surface.
However, the super hard alloy or cermet base member is a sintered body mainly consisting of tungsten carbide (WC), titanium carbide (TiC), alumina (Al.sub.2 O.sub.3), titanium nitride (TiN), chromium carbide (Cr.sub.3 C.sub.2) or other ceramics, sintered by using, as a binder, an active metal such as cobalt (Co), nickel (Ni) and molybdenum (Mo). These active metals, during long-time use of the mold, would diffuse through the noble metal layer to the mold surface, and react with the glass to deposit on the mold surface. As a result, the shape precision of the mold surface would be spoiled, to lower the optical performance of the press-molded optical glass elements.